1. Field of the Invention
The present invention relates to a forging technology, and in particular, relates to a press forging technology wherein a round billet is used as a raw material.
2. Description of the Related Art
In a forging process of the prior art, a rolled steel is used as a raw material. In other words, in a forging process of the prior art, it requires a rolling process as a pretreatment process.
This is because, in rolled steel, porosities (blow holes and others) are removed in the rolling process.
Meanwhile, there are various kinds of production lines. For example, there is a production line wherein:
press forging machines suitable for press forging a steel ingot such as a round billet are provided;
a press forging process being directly applied to a steel ingot, which is treated as a raw material, is required; and
a rolled steel is not processed.
However, in the case that a steel ingot is treated as a raw material, it is necessary to remove porosities found in the ingot. The reason is that, if there are porosities in the raw material, ductility and toughness of the steel products are deteriorated.
In the case that a steel ingot is treated as a raw material in the prior art, the conditions for applying force to a steel ingot and a forging ratio thereto are preliminarily determined and it is necessary to satisfy such conditions (the predetermined conditions).
Further, in the prior art, in the case that a steel ingot is treated as a raw material, killed steel has been used and portions, in which there are porosities and segregation, have been cut off.
If the raw material described above is used, the process of cutting off the portions, in which there are porosities and segregation, is required. Moreover, a yield of a steel product is deteriorated because of the cutting off of portions of the steel ingot.
As mentioned above, in the prior art, it is difficult to treat a steel ingot as a raw material, and a yield of a steel product is deteriorated. Therefore, in the case that a low-price steel ingot has been treated as a raw material, it is impossible to take advantage of the cost advantages of using “a low-price steel ingot”.
The relationship between porosity or segregation and a reduction ratio or the like has heretofore been studied (for example, refer to 1) “Seitetsu Kenkyu,” Vol. 309, “Metallurgical significance of hot-rolling of continuously-cast steel on steel plate quality” Authors: Michihiko Nagumo, Naoki Okumura, and Yasushi Inoue; 2) “Tetsu To Hagane,” 1980, Vol. 2, “Influence of rolling conditions on the elimination of porosities in a continuously-cast slab” Authors: Naoki Okumura, Takeshi Kubota, Tadakatsu Maruyama, and Michihiko Nagumo; and 3) Japanese Standards Association, “JIS Handbook 2006, Steel I,” pp. 548, “General rule on production, test, and inspection of forged steel product”).
However, in the above-mentioned documents, there are no explanations for removing porosities and ameliorating ductility and toughness of a steel product in the case that a steel ingot is used as the raw material.
Further, in the prior art, there is a known technology for producing a thick steel plate having excellent internal properties. In such technology, there are steps for solidifying a steel in a mold, for removing the steel product from the mold as soon as the solidification of the steel, for hot-rolling the steel, and for applying light reduction to the steel in the thickness direction during the hot-rolling process.
However, such the prior art (Japanese Patent Publication No. S62-134101) does not disclose any ways for solving the above-mentioned problems.